The present invention relates to improvements in apparatus for measuring the diameters of moving round objects, and more particularly to improvements in apparatus for monitoring the diameters of objects whose axes orbit about a fixed axis. Typical examples of such objects are the crankpins of a crankshaft which is installed in a machine tool (e.g., in a grinding or milling machine) for rotation about an axis which is the common axis of its two end portions and is parallel to the axes of the crankpins.
Monitoring apparatus of the above outlined character normally comprise a detector (also called measuring head) which is provided with one or more finger- or prong-like sensors in contact with the periphery of the circulating workpiece. Such monitoring apparatus operate quite satisfactorily as long as the axis of rotation of the object (e.g., a workpiece) coincides with the axis of that part which is being treated, i.e., of the part whose diameter is in the process of diminishing as a result of removal of material therefrom by one or more grinding wheels, milling cutters or other suitable tools. However, the situation becomes much more complicated when a selected portion of a workpiece (such as a selected crankpin of a crankshaft with two or more crankpins) must circulate about a fixed axis which does not coincide with the axis of the selected portion of the workpiece. Under such circumstances, conventional monitoring apparatus cannot ensure continuous contact between the sensor or sensors and the periphery of the selected portion of the workpiece so that the monitoring operation is unreliable and the removal of material from the selected portion of the workpiece is likely to be terminated prematurely or too late.